This section introduces aspects that may be helpful in facilitating a better understanding of the invention. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is known in the art.
In a massive Multiple-Input Multiple-Output (MIMO) communication system a massive MIMO central node, sometimes also referred to as massive MIMO base station, is provided with an antenna array comprising multiple, e.g. M, antennas for serving a plurality of, e.g. K, terminals over the same time/frequency resources using spatial multiplexing. A key factor for successful operation of the multiplexing operation in the massive MIMO communication system relates to the acquisition of sufficiently accurate channel state information (CSI). In particular the massive MIMO base station needs to obtain sufficiently accurate estimates of the frequency responses of the propagation channels between each of its M antennas and each of the K terminals.
Obtaining such CSI is relatively easy for a massive MIMO communication system employing time division duplex (TDD), where the same frequency band is utilized for both uplink and downlink data transmission. CSI may be acquired by simultaneous transmission of orthogonal pilot sequences by the K terminals to the M base station antennas, from which the uplink propagation channel state between each of the M base station antennas and the K terminals is estimated. By virtue of reciprocity, the uplink channels are equal to the downlink channels. The sample duration is independent of the number of base station antennas, and execution of a training scheme to obtain the required propagation channel state estimates is relatively fast as the entire process for K terminals requires K resource samples, and in a typical MIMO system the number of terminals is relatively low as compared to the number of base station antennas.
However, in Europe and North America most wireless communication systems operate using frequency-division duplex (FDD), where uplink and downlink channels are typically located in different frequency bands. Under FDD, the transmission of uplink orthogonal pilot sequences still suffices for the MIMO base station to estimate the state of the uplink propagation channels. However, a further approach is needed for the MIMO base station to obtain sufficiently accurate estimates of the state of the downlink propagation channels, which is generally time-consuming.
Considering a system with M base station antennas and K single-antenna terminals, a known way to acquire the downlink CSI is for the K terminals to send pilot signals towards the M base station antennas, which enable the MIMO base station to obtain the CSI of the uplink channels. Additionally, to obtain the downlink CSI, the M base station antennas simultaneously transmit orthogonal pilot sequences on the downlink, and each of the K terminals receive the combination of pilots through the propagation channels. Each terminal then, in real time, simultaneously transmits its analog composite received pilot signal back towards the base station on the uplink. Through signal processing, and its knowledge of the uplink channels the base station may then reliably estimate the downlink channel states. The entire process requires a minimum of 2M+K resource samples. As the number of base station antennas M is typically much higher than the number of terminals K in a massive MIMO system obtaining reliable CSI for the uplink and downlink propagation channels in a MIMO-system employing FDD is much more cumbersome than obtaining such CSI in a MIMO-system employing TDD.
The article entitled “Pilot-Assisted Channel Estimation Method for OFDMA Systems over Time-Varying Channels” by Li et al. in IEEE Communications Letters, Vol. 13, No. 11, pp 826-828 describes approximation of time variations of a frequency domain transmission function in one symbol period by a linear model in time-frequency blocks for each user involved.
US-patent application publication 2014/0219377 describes channel estimation for a very large-scale MIMO system in which a transmitter includes a multiplicity of antennas spaced such that spacing between adjacent antennas provides a spatial correlation coefficient greater than a threshold level. The transmitter selects a subset of the transmit antennas for transmitting pilot reference signals to a receiver. The pilot reference signals are transmitted only from the selected subset. The receiver includes a channel estimator configured to derive a channel estimation for all of the transmit antennas using the received pilot reference signals and known or estimated spatial correlation among the transmit antennas.
It is desirable to reduce the time for obtaining the CSI in a FDD system, in particular for MIMO-systems with a relatively large number of terminals.